During the three periods of observation there was no general deposit of dew on the 

 ground, but one day in August there was dew on the wind gauge, and on twigs of vari- 

 ous dried up plants on the plateau above the wady. On this morning the dew point at 

 5 a.m. was 17°C and the surface soil 17.5°C, so that a very small further fall of tempera- 

 ture would have produced a general ground dew. It is interesting to note in this con- 

 nection that a desert plant, Reamuria hirtella J. and S. of the family Tamaricacea, 

 which was not uncommon in some spots along the wady, was found to be dripping wet 

 in the early morning whenever the relative humidity of the air was above 75%. This 

 was found to be caused by small crystals of sodium chloride on the surface of the plant 

 which absorbed moisture from the atmosphere above this relative humidity. 



It will be clear from the above that within a distance of relatively few metres there 

 are available in this type of desert country a very wide range of temperature conditions 



25 



